Shaken, not stirred, bubble column reactors: enhancement of mass transfer by vibration excitement

Abstract

In this paper we discuss the advantages of a bubble column reactor, wherein the liquid phase is subjected to low-frequency vibrations. A special device, called a vibration exciter, mounted at the bottom of the bubble column, transmits the vibration to the liquid phase by means of a piston. Both the amplitude of the vibration, and its frequency can be adjusted quite accurately. We show that application of low-frequency vibrations, in the 40-120 Hz range, to the liquid phase of an air-water bubble column causes significantly smaller bubbles to be generated at the nozzle. In experiments with a single capillary nozzle the bubble size is reduced by 40-50% depending on the gas flow rate. Using a 12-capillary nozzle arrangement, the gas holdup, ε, and the volumetric mass transfer coefficient, k_La, were measured for a range of superficial gas velocities. Application of vibrations to the liquid phase leads to enhancement in ε and k_La values by a factor of two or more. The increase in the gas holdup is attributed mainly to a significant reduction in the rise velocity of the bubble swarm due to the generation of standing waves in the column. Furthermore, application of vibrations to the liquid phase serves to stabilize the homogeneous bubbly flow regime and delay the onset of the churn-turbulent flow regime. It is concluded that application of low-frequency vibration has the potential of improving the gas-liquid contacting in bubble columns.